Even though the use of anti-D immunoglobulin has dramatically decreased the incidence of hemolytic disease of fetus and newborn, it still remains a significant cause of fetal and neonatal morbidity and mortality. The main challenge facing fetal medicine specialists today is not the skill required for invasive therapy, but rather the noninvasive monitoring of the disease so that its progress can be predicted to guide the need and timing of intrauterine transfusions to minimize unnecessary invasive testing. In previous years many different diagnostic tests were proposed but the assessment of middle cerebral artery peak systolic velocity still stands as a gold standard for noninvasive assessment of fetal anemia.